This invention relates to capacitors for detecting voltages applied to components in a vacuum enclosure.
Mass spectrometers are instruments for analyzing the chemical composition of matter, usually by using electric or magnetic fields to separate ionized particles by their mass-to-charge ratio. For example, in a quadrupole mass spectrometer, the separation is achieved by driving two pairs of electrodes, one pair driven with a selected radio frequency RF voltage summed with a positive direct current DC voltage, and the other pair driven with the 180xc2x0 out-of-phase RF signal summed with a negative direct current (DC) voltage.
Accurate measurement of the level of RF voltage driving the electrodes is vital to the success of mass spectrometers, both to provide feedback to the RF circuit to ensure the level of RF is the desired level, and to provide the information to the DC control circuit to ensure that the desired ratio of RF to DC voltage is maintained.
Detector capacitors are used to detect the input RF voltage applied to the pairs of rods and provide an output current which is representative of the input RF voltage. The output current is applied to RF and DC control circuits which control the RF and DC input to the mass spectrometer.
Prior art detector capacitors operate at atmospheric pressure and incorporate features which ensure that the capacitance does not change with temperature changes. One prior art design of a detector capacitor uses low thermal coefficient of expansion metal (e.g. Invar composition 36% Ni balance iron) with insulating parts made of low thermal coefficient of expansion insulator (e.g. Quartz or Alumina). Another prior art design uses materials where the coefficient of expansion are arranged so that changes in dimensions act to cancel any change in capacitance.
The present invention provides a capacitor which is adapted to be mounted on the wall of a vacuum enclosure, such as the manifold of the mass spectrometer evacuation system, with the capacitor plates inside the enclosure, so that the space between the plates is evacuated by the vacuum system, and a feed-through provides a connection to one of the capacitor plates.